The present invention relates to a composite disc for a disc brake with a braking which can be split into at least two portions.
Composite disc brakes of the type indicated above are known and are particularly suitable for use on heavy vehicles for both road transport (lorries, commercial vehicles) and rail transport; in these disc brakes, the braking band comprises two portions which are connected to one another releasably so that they can be replaced when their braking surfaces are completely worn out, without the need to remove the bearing unit from the stub axle of the wheel. The need to use brake discs with easily removable braking bands is dictated by the fact that bearing manufacturers now guarantee a life which is much longer than that of brake discs, but upon the condition that the bearings are never demounted after the original assembly. The use of brake discs having braking bands which can be split into at least two portions has therefore been proposed for this purpose.
For a better understanding of the state of the art on the subject in question and of the problems inherent therein, two known solutions for the production of a composite disc-brake disc provided with a braking band which can be split into two portions will be described first of all.
In the description and in the appended claims, terms such as “axial”, radial, and “tangential” are intended to refer to the condition of mounting on a vehicle or, preferably, to the axis of symmetry and/or of rotation of the disc.
A first embodiment of a disc brake of the type specified above is described in German patent application DE 2 301 246 from which FIG. 4 of the appended drawings is taken. With reference to this drawing, a braking band 3 of a composite ventilated disc 1 for a disc brake usable particularly in rail transport comprises a first portion 3a and a second portion 3b which are separate from one another. These portions, which have coupling surfaces lying in an axial dividing plane π, are placed side by side at the level of respective flat coupling surfaces 4a and 4b defined by this plane and are then joined together by tangentially-oriented, threaded connecting members 8. To ensure alignment between the two portions 3a, 3b of the braking band 3, two locating pins 6, also oriented tangentially, are inserted in respective cylindrical seats formed in the flat coupling surfaces 4a, 4b. 
The main disadvantage of this solution is that it is impossible to ensure precise alignment between the two portions of the braking band, particularly axially, precisely because the task of ensuring this alignment is entrusted to a pair of tangential pins. The use of tangential locating pins in fact requires that their seats in the flat joining surfaces 4a, 4b be produced independently of one another. Owing to the working tolerances associated with the drilling of these seats, they are inevitably not positioned at the same distance (axially) from the two axially-opposed faces of the respective portions 3a, 3b. Thus, if the two portions of the band are assembled with the use of a pair of pins inserted in the above-mentioned tangential seats as a reference for their mutual alignment, the respective braking surfaces are not perfectly aligned with one another, particularly axially. That is, these surfaces do not define a single braking plane, but a step-like discontinuity arises between one surface and the other along the line of the join between the two portions of the band and, during braking, acts as a tool, removing material from the braking surfaces of pads housed in calipers, thus causing the pads to wear out rapidly.
A second method of producing a composite disc provided with a braking band which can be split into two portions is known from international patent application WO 93/21453; FIG. 5 of the appended drawings, in which parts and elements identical or corresponding to those of FIG. 4 have been attributed the same reference numerals, is taken from this application. In this case, the two portions 3a, 3b of the braking band 3 are formed by splitting in half, along a substantially diametral breakage line P, a band which has previously been produced by casting in a single piece. The band 3 is assembled by fitting the matching brittle breakage surfaces 4a, 4b of the two portions together and securing them by means of tangentially-oriented, threaded connection members 8.
However, this known solution is subject to the problem that it is difficult to control errors of shape (mainly the axial variation of the braking surfaces) which inevitably occur as a result of the deformations that are produced during the splitting of the band and which may lead to a considerable number of rejects in the manufacturing process.